from greenlet import greenlet
import  time

def test1():
    while True:
        print('---A---')
        gr2.switch()
        time.sleep(0.5)

def test2():
    while True:
        print('---B---')
        gr1.switch()
        time.sleep(0.5)


gr1=greenlet(test1)
gr2 =greenlet(test2)
gr1.switch()
